In the normal human heart, the sinus node, generally located near the junction of the superior vena cava and the right atrium, constitutes the primary natural pacemaker initiating rhythmic electrical excitation of the heart chambers. The cardiac impulse arising from the sinus node is transmitted to the two atrial chambers, causing a depolarization known as a P-wave and the resulting atrial chamber contractions. The excitation pulse is further transmitted to and through the ventricles via the atrioventricular (A-V) node and a ventricular conduction system causing a depolarization known as an R-wave and the resulting ventricular chamber contractions.
Disruption of this natural pacemaking and conduction system as a result of aging or disease can be successfully treated by artificial cardiac pacing using implantable cardiac stimulation devices, including pacemakers and implantable defibrillators, which deliver rhythmic electrical pulses or anti-arrhythmia therapies to the heart at a desired energy and rate. A cardiac stimulation device is electrically coupled to the heart by one or more leads possessing one or more electrodes in contact with the heart muscle tissue (myocardium). One or more heart chambers may be electrically stimulated depending on the location and severity of the conduction disorder.
A stimulation pulse delivered to the myocardium must be of sufficient energy to depolarize the tissue, thereby causing a contraction, a condition commonly known as “capture.” In early pacemakers, a fixed, high-energy pacing pulse was delivered to ensure capture. While this approach is straightforward, it quickly depletes battery energy and can result in patient discomfort due to extraneous stimulation of surrounding skeletal muscle tissue.
The capture “threshold” is defined as the lowest stimulation pulse output (as may be reported in terms of pulse duration, pulse amplitude, pulse energy, pulse current or current density) at which consistent capture occurs. By stimulating the heart chambers at or just above threshold, comfortable and effective cardiac stimulation is provided without unnecessary depletion of battery energy. Threshold, however, varies significantly from patient to patient due to variations in electrode systems used, electrode positioning, physiological and anatomical variations of the heart itself, and so on. Furthermore, threshold will vary over time within a patient as, for example, fibrotic encapsulation of the electrode occurs during the first few weeks after surgery. Fluctuations may even occur over the course of a day or with changes in medical therapy or disease state.
Hence, techniques for monitoring the cardiac activity following delivery of a stimulation pulse have been incorporated in modern pacemakers in order to verify that capture has occurred. If a loss of capture is detected by such “capture-verification” algorithms, the pacemaker output is automatically increased until capture is restored. A threshold test is then performed by the cardiac stimulation device in order to re-determine the threshold and automatically adjust the stimulating pulse output. While a primary parameter to vary for adjusting the stimulation pulse output is the voltage, it should be clear that other parameters could be adjusted as well, including pulse duration, energy, charge, and/or current density.
This approach, referred to as “automatic capture,” improves the cardiac stimulation device performance in at least four ways: 1) by verifying that the stimulation pulse delivered to the patient's heart has been effective, 2) by maintaining the stimulation pulse output at the lowest level possible, thus 3) greatly increasing the device's battery longevity by conserving the energy used to generate stimulation pulses, yet 4) always protecting the patient by providing a significantly higher output back-up pulse in the setting of loss of capture associated with the primary pulse.
One implemented technique for verifying capture automatically by an implantable stimulation device involves monitoring the intra-cardiac electrogram signal, also referred to as EGM or IEGM, received on the cardiac stimulation and sensing electrodes. When a stimulation pulse is delivered to the heart, the EGM signals that are manifest concurrent with depolarization of the myocardium are examined. When capture occurs, an “evoked response” may be detected by special evoked response detection circuitry. The evoked response is the intracardiac atrial or ventricular depolarization that is observed as the P-wave or R-wave, respectively, on the surface ECG associated with a stimulus output. Detection of an evoked response indicates electrical activation of the respective cardiac tissue by the stimulating pulse. The depolarization of the heart tissue in response to the heart's natural pacemaking function is referred to as an “intrinsic response.”
Through sampling and signal processing algorithms, the presence of an evoked response following a stimulation pulse is determined. A very short blanking period, or period of absolute refractoriness, following the stimulation pulse is applied to the evoked response sensing circuit immediately following the stimulation pulse to minimize or block out the stimulation pulse artifact.
This blanking period is followed by a special evoked response detection window, commonly 15 to 60 ms in duration, wherein the evoked response sensing circuit looks for an evoked response. For example, if a stimulation pulse is applied to the ventricle, an R-wave sensed by a special evoked response detection circuit of the pacemaker immediately following application of the ventricular stimulation pulse evidences capture of the ventricles.
If no evoked response is detected, a high-energy back-up stimulation pulse is delivered to the heart very shortly after the primary ineffective stimulus, typically within 60-100 ms of the primary pulse, in order to maintain the desired heart rate. If back-up stimulation pulses are required on two successive cycles, the system automatically begins to increase the stimulation output associated with the primary pulse until capture is restored, again for two consecutive cycles. Once capture is regained, an automatic threshold test is performed to re-determine the minimum pulse energy required to capture the heart at that time and adjust stimulation pulse output as needed.
An exemplary automatic threshold determination procedure is performed by progressively reducing the output from the functional output in 0.25 Volt steps until loss of capture occurs. With each loss of capture, a higher output back-up pulse is delivered in order to maintain the desired heart rate. Once loss of capture is achieved, the system increases the output in 0.125 Volt steps until stable capture is restored. Stable capture is defined as capture occurring on two consecutive primary pulses. Thus, reliable capture verification is of utmost importance in proper determination of the threshold.
Normally, capture threshold should be stable after the initial postoperative healing period. Frequent fluctuations in threshold can occur later, however, if a stimulating lead becomes dislodged, fractured, or its insulting sheath becomes discontinuous. Fluctuations in threshold may also reflect a change in clinical condition or the effects of a pharmacological agent. The automatic capture feature responds to such fluctuations by repeating a threshold test whenever the threshold rises enough to cause a loss of capture at the existing output setting. Threshold tests may also be repeated on a periodic basis to ascertain if a decrease in threshold has occurred. This automatic feature protects the patient by ensuring adequate stimulation pulse energy despite fluctuating threshold.
It is also desirable to store threshold test results on a frequent basis. Having a record of threshold changes over time will alert a medical practitioner to a possible lead failure or a change in the clinical condition of the patient, both of which warrant further medical evaluation. Such a feature is also referred to herein as “long term threshold record.”
Sensing an evoked response during threshold tests or capture verification, however, can be difficult for several reasons. One confounding factor in accurate capture detection is “fusion.” A fusion event occurs when a native depolarization and a stimulation pulse combine to contribute to the overall depolarization. The R-wave resulting from such a fusion event may be considerably diminished and not detectable by the normal R-wave detection scheme used for automatic capture verification. A fusion event may therefore easily be mistaken for a loss of capture.
Another somewhat related event that can confound accurate capture detection is “pseudofusion.” Pseudofusion occurs when a stimulation pulse is delivered simultaneously with an intrinsic depolarization but in this case does not contribute at all to the overall depolarization. The resulting R-wave, however, is distorted as observed on the EGM resulting in a loss of capture detection when in fact a native R-wave has occurred.
In each of the above cases, the result may be a loss of capture detection by the cardiac stimulation device when in fact a native depolarization prevented the algorithm from recognizing that there was a successful cardiac depolarization. The loss of capture detection will cause the stimulation device to deliver a high-energy back-up stimulation pulse and invoke the threshold testing function in a chamber of the heart even though these actions are not clinically necessary.
To overcome the problem of fusion, and to prevent the intrinsic heart rhythm from interfering with the process of stimulation and capture during threshold testing, the time-out interval after which a stimulation pulse is delivered is commonly shortened in order to ensure stimulation occurs before a native depolarization. During single chamber stimulation, the escape interval is shortened; during dual chamber stimulation, the AV and PV delays are shortened. The AV delay is the interval following an atrial stimulation pulse that precedes delivery of a ventricular stimulation pulse. The PV delay is the interval following an atrial sensed P-wave preceding a ventricular stimulation pulse. By shortening the AV and PV delays, the ventricular pulse is delivered earlier than normally programmed following an atrial stimulation pulse or atrial sensed P-wave, respectively, and is expected to precede any natural depolarization of the ventricles.
However, in some patients, a shortened AV or PV delay can cause deleterious hemodynamic effects, which may contribute to adverse symptoms or hypotension. Furthermore, in patients with first degree or even more severe atrial-ventricular conduction block, the shortened AV and PV delays are not required in order to perform a threshold test without encountering fusion. Thus, automatic obligatory shortening of the AV and PV delays to a non-physiologic interval may be unnecessary and undesirable.
Another signal that interferes with the detection of an evoked response, and potentially the most difficult for which to compensate because it is usually present in varying degrees, is lead polarization. A lead-tissue interface is that point at which an electrode of the pacemaker lead contacts the cardiac tissue. Lead polarization is commonly caused by electrochemical reactions that occur at the lead-tissue interface due to application of an electrical stimulation pulse across the interface. If the evoked response is sensed through the same lead electrodes through which the stimulation pulses are delivered, the resulting polarization signal, also referred to as an “afterpotential”, formed at the electrode can corrupt the evoked response signal that is sensed by the sensing circuits. This undesirable situation occurs often because the polarization signal can be three or more orders of magnitude greater than the evoked response. Furthermore, the lead polarization signal is not easily characterized; it is a complex function of the lead materials, lead geometry, tissue impedance, stimulation energy and other variables.
Before enabling the automatic capture function of a cardiac stimulation device, an automatic capture calibration algorithm is performed which includes an evoked response sensitivity test. During an evoked response sensitivity test, the amplitude of the sensed evoked response and the amplitude of the lead polarization signal are measured. The test is performed by delivering stimulation pulse pairs at a high output setting, typically 4.5 Volts. This high output setting provides a “worst case” scenario in terms of the lead polarization signal. The first stimulation pulse will capture the myocardium producing an evoked response signal. The second stimulation pulse is delivered shortly after the first pulse when the myocardium is physiologically refractory and capture is impossible. The second pulse will therefore produce only a lead polarization signal without an evoked response. The evoked response amplitude measured after the first pulse is then compared to the polarization signal amplitude measured after the second pulse.
The difference between these signals must meet a minimum requirement so that the automatic capture verification can reliably distinguish between an evoked response and pure polarization in order to appropriately recognize capture and loss of capture. Based on the difference between the evoked response signal and the polarization signal, the cardiac stimulation system can automatically recommend whether automatic capture should be enabled and, if so, provide a recommended evoked response sensitivity setting.
Just as during threshold testing the evoked response sensitivity test is typically performed at shortened AV and PV delays so that an accurate measurement of the evoked response amplitude may be made without interference from native R-waves. While this situation eliminates the problem of fusion for measurement purposes, it may not reflect the normal day-to-day operating conditions of the stimulation device. Automatic capture verification will be enabled with settings that are valid at the tested AV and PV delays but may not be the optimal settings at the final programmed AV and PV delay settings.
It would thus be desirable to provide an implantable dual-chamber or multi-chamber cardiac stimulation device possessing automatic capture in which threshold testing and evoked response sensitivity measurements are performed in a way that avoids the possibility of an adverse hemodynamic response in an individual patient and further provides realistic results regarding the day-to-day performance of the device. Further, it would be desirable to provide a threshold record capable of documenting fluctuations in threshold that occur over brief periods of time so that a clinician may better monitor lead stability or identify changes in clinical condition.